1. Field of the Invention
The invention relates to a method for monitoring printed material which is produced by a printing press and comprises one or more images printed on a substrate, in particular a paper web.
2. Description of the Related Art
In a printing process, for instance a rotation offset printing process, a large number of identical copies is made on the basis of an original, for instance an illustration or a text. It is of great importance here that the images in the printed material are transferred to the substrate for printing, generally a paper web, as far as possible in colour-true, correctly positioned and consistent manner. “Colour-true” is here understood to mean that the printed colours correspond precisely with the colours of the original which is being duplicated. This is especially important for advertisers, who increasingly use a colour as a mark and therefore wish to see it printed correctly. The correct position is important because it determines the general picture of the printed material, while consistency is important in order to indeed be able to ensure identical copies of the printed material.
A problem which occurs here is that the final colour of a printed image is influenced by a large number of variables which cannot be fully monitored during printing. Differences will thereby occur between the original and the printed images. In order to be able to understand the causes of these differences, some insight into the nature of printed materials is required.
Printed materials are built up of grid points or dots. The printed image is determined by two quantities, the number of grid points per unit of length, usually expressed in “dpi” (dots per inch), which specifies how fine or crude the image is, and the relation between light and dark, the tonal range. As long as the tonal range takes on values of less than 50%, the image is formed by dark, thus printed dots on a light background, while for higher values use is made of light dots, thus dots not printed in the surrounding, printed background.
During the different steps which must be performed during the printing process to form a printed image from an original, there do however occur differences in the size of the grid points; the points grow, designated as “dot gain” or shrink, designated as “dot loss”. The final printed image hereby differs from the original. The degree to which this occurs depends on a large number of variables associated with the printing process, and on the original size of the grid points. The finer the printed material is, i.e. the closer together the grid points, the greater the differences are. Furthermore, the differences are often relatively greatest in parts of the printed material where the tonal range amounts to about 50 percent.
In practice therefore, printing presses are calibrated before use. Proofs are made with different finenesses and different tonal ranges, and the differences are determined from these proofs. These measured differences are stored in the form of calibration graphs, so-called dot gain curves. These graphs are used to determine the theoretical optimal settings of each printing press when an original with a determined fineness and tonal range is presented.
As stated, there are many variables whereby the quality of the final printed material is influenced. Envisaged here are the type of paper used, the water-ink balance, the temperature, possible contamination of the ink or the ink rollers, the pressure between the different ink rollers, vibration occurring in the printing presses and the like. A change in just one of these quantities can result in an increase or decrease in size of the grid points, and therefore in differences in the printed colours. Because it is almost impossible to hold all these quantities constant during the printing process, different methods have been developed to control these quantities on the basis of inspection of the colours such that the quality remains constant.
There are thus so-called open loop control methods, wherein random samples of the printed material are monitored using hand-held measuring devices. These hand measuring devices monitor a separately co-printed colour bar. When differences are detected an operator can change the settings of the printing press. A drawback of this control method is the discontinuous and even informal nature thereof. Between two consecutive random samples a large quantity of printed material of substandard quality can be produced. This method moreover requires the presence of an often expensive operator.
Fully closed control methods have therefore been developed, wherein measuring and control systems monitor the quality of the printed material fully autonomously and adjust the settings of the printing press when differences are detected. These known systems are also based on a co-printed colour bar. There are two major trends here. Known in the first instance are systems wherein the colour bar consists of areas which are completely covered with the basic colours C (cyan), M (magenta), Y (yellow) and K (black), whereby the colour density (thickness of the ink layer) can be measured. In improved systems the colour bar also comprises grey areas, including a black area with 50% tonal range and an area with cyan, magenta and yellow in quantities such that black printed material with 50% tonal range is likewise obtained. Differences in the size of the grid points of cyan, magenta and yellow can be determined by comparing these areas. These closed control methods also have the drawback that a separate colour bar has to be co-printed for this purpose. If this colour bar is not cut away, the appearance of the printed material is impaired. The co-printed colour bar is otherwise also used to monitor the positioning of the printed material on the paper web and/or to monitor the relative location (register) of the different colours, as described in the older European patent 0 850 763 of applicant.
A drawback of both the open and closed control methods is that the printing press must be properly adjusted so as to ensure that a correctly printed colour bar also actually means that the total printed image is correct, since the colour bar is situated outside the normal printed image, and usually also differs greatly from the average printed image in respect of the colours used therein.